Temperature sensing devices have previously been proposed which use a semiconductor device in the form of a diffused resistor formed by diffusing into a semiconductor body of one conductivity type impurities of the opposite conductivity type providing first and second electrodes spaced-apart on the diffused region so that the diffused region provides a resistive path between the first and second electrodes. Such diffused resistors have a positive temperature coefficient of resistance, that is their resistance increases with temperature and can thus be used to sense temperature.
U.S. Pat. No. 4,092,662 describes a temperature sensitive resistor in which the temperature coefficient of resistance is selected by selecting the doping of the diffused resistor. The diffused resistor is formed in an isolation well and one of the resistor electrodes is electrically shorted to the isolation well. Several of these resistors may be connected in series to enable control of the voltage coefficients of resistance.
U.S. Pat. No. 5,039,878 (EP-A-369530) describes an example of a temperature sensing circuit in which the voltage across a diffused resistor temperature sensing device is compared with that across a second temperature sensing device having a negative temperature coefficient of resistance, for example a diode, to provide a more accurate indication of the sensed temperature.
Diffused resistors do however tend to have a temperature coefficient of resistance which varies significantly with temperature. In particular, the temperature coefficient of resistance of a diffused resistor tends to fall or decrease with temperature, that is the increase in resistance with temperature becomes smaller at higher temperatures, which can present problems and lead to less accurate temperature sensing in some applications.